1. Field of the Invention
The present invention relates to an ion selective electrode, and in particular relates to a sodium ion selective electrode.
2. Description of the Related Art
Conventionally, a polarography method and an atomic absorption spectrometry method were used to detect concentrations of potassium ions and sodium ions in blood. However, these methods require a pre-treatment, which makes the methods inconvenient to use. Ion sensitive field effect transistors (ISFET) having a silicon nitride (Si3N4) film is an improvement over metal insulating field effect transistors (MISFET) having a metal oxide film. For an ISFET, a silicon nitride (Si3N4) film with active substances is used as the ion sensing film and an Ag/AgCl is used as a reference electrode. Note that the ISFET sensor may be directly placed in a sample solution. When the substances in the sample solution reacts with receptors on the ion sensing film, a zeta potential of the ion sensing film is changed and a signal is acquired from drain of FET. An ISFET may be used to sense different kinds of ions by selecting different ion selective materials on the sensing film. Disclosed ISFETs can sense Na+, K+, NH4+, Ca2+, Ag+, Li+, Cl− and Br−, respectively.
Enzyme field effect transistors (EnFET) replace the ion sensing film of ISFETs with enzyme immobilized film. Conventionally, EnFETs were used for detecting penicillin, glucose or acetylcholine. The advantages of the ISFET comprise: (1) miniaturized size, which may be micro processed by a semiconductor technique; (2) multiplication, which allow processing as a sensor array to detect various compositions; (3) fast response, which reduces response time.
In 1983, J. Van der Spiegel et al. developed an extended gate chemical sensitive field effect transistor, which uses a plane array structure having four sensing parts deposited with different materials such as IrOx, LaF3, AgCl and Ag2S which form the sensing thin film for detecting four kinds of ions, H+, F−, Cl− and Ag+.
Separative extended gate ion sensitive field effect transistors (SEGFET) are another kind of ISFET. Compared to conventional ISFETs, SEGFETs have advantages such as more simplified packaging, easier preservation, less light influence, less limitations on shape and size and better stability.